Module documentation for 0.1.0

It is similar to TimerManager from GHC.Event, but can scale much better
under concurrent access patterns.

An unreleased version of this library has a backpack-based signature for
selecting from a variety of different data structures used internally.
However, until backpack has broader support on Hackage, and in Haddocks,
etc, I decided to just hard-code something decent (a priority search queue).
This makes the O(1) claims above a bit of a lie, as these rely on using a
linked list data structure internally.